Method for producing a steering-wheel cover

ABSTRACT

For the production of a steering wheel with a seamless casing, surrounding the steering wheel rim, the steering wheel spokes and the central region for an airbag covering with tear-open lines, of a plastic moulded part produced in one piece by casting or injection moulding, with a closed surface and a possibly foamed base body, a method is proposed which is characterized in that a core of a low-melting material is fastened to the central base part of the steering wheel skeleton structure, that thereafter the steering wheel skeleton structure and the core are placed into a mould for the formation of the plastic moulded part, that the plastic moulded part is produced and that after the setting of the plastic moulded part, the core is melted by an increase in temperature and the material is removed in liquid form from the plastic moulded part.

FIELD OF THE INVENTION

The invention relates to a method for the production of a steering wheelwith a seamless casing which surrounds the steering wheel rim, thesteering wheel spokes and the central region of the steering wheel whichcomprises an airbag covering with tear-open lines. The steering wheelcasing is a plastic moulded part produced in one piece by casting orinjection moulding, with a closed surface and a possibly foamed basebody.

BACKGROUND OF THE INVENTION

The majority of casings of steering wheels with an integrated airbag areconstructed today in two parts. A moulded part surrounding the skeletonstructure of the steering wheel and with a separately produced coveringfor the airbag installation opening in the hub region of the steeringwheel, with great efforts being made to match the two parts to eachother visually and haptically. Considerable further difficulties resultfrom the fact that the airbag covering has to be constructed so that thecovering is able to be torn open and is also movable relative to thesteering wheel hub by the touch stroke necessary for actuating the horn.

As a gap between the moulded part encompassing the steering wheel andthe airbag covering is regarded as being visually disadvantageous, ithas already been proposed to produce the moulded part and airbagcovering in one piece. Owing to the tear-open and actuation functions tobe taken into account in the region of the airbag coverings, suchmethods involve certain substantially solvable difficulties with regardto manufacturing technique and compulsorily require a largely new designof the steering wheel construction, because the installation can onlytake place from the rear side of the steering wheel and because specialprecautions have to be taken for the introduction of the airbag modulewith folded gas bag and gas generator. One known design involvesdividing the steering wheel skeleton structure into a base part with huband possibly spoke stumps and a steering wheel rim with spokes. The basepart can be equipped with the airbag module and other functional parts,before the steering wheel rim is attached to the base part and theentire skeleton structure is covered by a one-piece sheathing.

It is regarded as particularly disadvantageous here that the fasteningbetween base part and steering wheel rim involves an additional materialexpenditure and adds weight to the structure, which counters the generalaim for saving weight in automobile construction. In addition, it isdifficult to match the two components optimally to each other withregard to dimensions. In particular, the gas bag covering, as a mouldedpart, is provided with considerable tolerances. Because the gas bagcovering has to be able to be torn, it must not have any particularinherent rigidity and therefore requires the folded gas bag as asupport. If the gas bag covering is too "thin", the covering is notsufficiently supported, if it is too "thick", it is buckled upwards.Both are undesirable.

Similar problems result with regard to the actuation movements whichhave to be transferred from the exterior via the plastic moulded part,which is constructed in one piece on the steering wheel rim, onto theactuating members mounted in the base part. It should be readily clearthat the adaptation of a surface, produced by casting mould technique,to a mounted surface is very costly with regard to manufacturingtechnique, and that the transfer functions can only be carried outreliably with regard to operation if very close tolerances can bemaintained in adaptation.

Resulting from this is the problem of further developing the methodinitially mentioned so that all the above-mentioned difficulties can beovercome and so that a one-piece plastic part for encasing a steeringwheel can be provided. with a justifiable economic expenditure.

SUMMARY OF THE INVENTION

To solve this problem, it is proposed according to the invention that acore of a low-melting material is fastened to the central base part ofthe steering wheel skeleton structure. Thereafter the steering wheelskeleton structure and the core are placed into a mould for theformation of the plastic moulded part. The plastic moulded part is thenproduced and after the setting of the plastic moulded part has occurred,the core is melted by an increase in temperature and the low-meltingmaterial is removed in liquid form from the plastic moulded part.

Through the use of a core of low-melting material, the plastic mouldedpart can be produced having an accurately dimensioned hollow space, intowhich subsequently the airbag module can be introduced. The airbagmodule itself cannot be used as the core, owing to the "soft" surface ofthe gas bag package, because the predefined fracture line necessary inthe covering region of the plastic moulded part can not be produced in areproducible manner. On the other hand, using a core of low-meltingmaterial in combination with the outer form, a very preciselydimensioned line of small wall thickness can be produced in the plasticmoulded part. The fracture line produced using the low-melting materialcore has a sufficient strength, close tolerances, and, upon inflation ofthe gas bag, tears open reliably and immediately. The production of thecovering region is thus just as simple as with a covering produced as aseparate component. Further, predefined tear lines can be realized whichare able to be stressed sufficiently in normal operation and are able tobe easily torn open in an emergency, just as reinforcement inserts andcomponents acting as a hinge. In addition, through the production in onepiece it is ensured that the steering wheel and covering regions havethe same appearance visually and haptically.

As material, removable by melting, for the core, in particular atin/bismuth alloy comes into consideration, melting at approximately114° C. However, suitable melting waxes or fluids can also be used,because the mould cores only have to have short service lives, andbecause the formation times for the plastic moulded parts are veryshort, i.e. are in the range of 120 to 240 seconds. Through an advancecooling process of the mould cores, the service life of such a wax orfluid core can be substantially increased.

Further advantages of the method according to the invention result fromthe fact that the easily melting material used for the cores can bealmost completely recovered on melting and can be used again for theproduction of cores.

The disclosed method can be adapted to the most varied of purposes ofuse and requirements. By the invention, accordingly, a method isproposed by which not only the difficulties initially mentioned can beovercome, but rather also is extremely flexible and reliable inoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention is explained in further detailwith the aid of the example embodiment illustrated in simplified form inFIGS. 1 to 3.

FIG. 1 shows a section through a steering wheel with melting core and

FIG. 2 shows a section through a steering wheel according to FIG. 1 withincorporated airbag module and

FIG. 3 shows a section through a steering wheel perpendicular to thesection according to FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a section is illustrated through a simplified steering wheel1, which comprises a steering wheel skeleton structure with spokes 2 anda hub part 2a and also a plastic moulded part 3, and the centralinstallation space of which is occupied by a core 5 able to be removedby melting. The material of the core 5 is melted by increasing thetemperature of the entire steering wheel 1 to a level which is higherthan the melting point of the core, but which is lower than the meltingpoint of the plastic molded part 3 so that the core material takes aliquid form which is easily removed from the plastic molded part. Thematerial of the core 5 preferably comprises a tin/bismuth metal alloyhaving a melting point of approximately 114° C. Alternatively, thematerial of the core 5 may be a wax having a melting point between 60°C. and 115° C., or a solidified fluid having a melting point between-10° C. and 10° C. The core 5 is aligned to the hub part 2a by means ofcore pins 7 and screws 6 and is then inserted with the skeletonstructure into a casting mould or injection mould, not illustrated, inwhich the plastic moulded part 3 is formed. The plastic moulded part 3,as usual, encompasses the steering wheel rim and the spokes 2. It has anintegrally formed covering cap 4 for the airbag module 12 (see FIG. 2),which after the removal by melting of the core 5 is installed in theresulting cavity. The plastic moulding part 3 is preferably producedusing a foamable polyurethane material. Alternatively, the plasticmolded part 3 could be produced using a two-step molding process whichresults in two layers of different thermoplastic materials, as is knownin the art.

The core 5 has a core ridge 8 for the formation of the predefinedfracture site in the covering cap 4, into which in a manner known per sea reinforcement insert 4a can be formed. The reinforcement insert 4a maybe made from a plastic fabric, a plastic net, a perforated metal foil,or a plastic film. The reinforcement insert 4a is omitted or interruptedalong the tear lines (not shown) in the central region of the plasticmolded part 3.

In FIG. 2, in the cavity 11 which results after the removal by meltingof the core 5 (see FIG. 1), an airbag module 12 is inserted and isfastened to the hub part 2a by means of the screws 9. In addition, asteering wheel rear covering 10 is illustrated in section, which islikewise fastened to the hub part 2a by means of the screws 6.Otherwise, FIG. 2 corresponds to FIG. 1; further explanations andreference numbers are therefore not necessary.

FIG. 3 shows a section through the steering wheel perpendicular to thesteering wheel axis, but rotated through 90° with respect to thesections according to FIGS. 1 and 2. Hence the opening 13 becomesvisible, which is provided between plastic moulded body 3 and steeringwheel skeleton structure 2, 2a, in order to be able to install theairbag module 12 into the cavity 11.

What is claimed is:
 1. A method for the production of a steering wheelcomprising a skeleton structure including a rim, spokes, and a basepart, said steering wheel further comprising a central region for anairbag covering and a seamless casing of a plastic molded part having acontinuous uninterrupted outer surface and a base body, the casingsurrounding said rim, said spokes, and said central region and havingtear-open lines, said method comprising the steps of:fastening a core ofa material having a melting point lower than that of said plastic moldedpart to said base part of said skeleton structure; placing said skeletonstructure and said core in a mould; forming said plastic molded part inone piece by casting or injection molding; setting of said plasticmolded part; melting said core by an increase in temperature to atemperature higher than said melting point of said core but lower thansaid melting point of said plastic molded part; and removing said corematerial in liquid form from said plastic molded part.
 2. The methodaccording to claim 1 wherein said core material comprises a tin/bismuthmetal alloy having a melting point of approximately 114° C.
 3. Themethod according to claim 1 wherein said core materials comprises a waxhaving a melting point between 60° C. and 115° C.
 4. The methodaccording to claim 1 wherein said core material comprises a solidifiedfluid having a melting point between -10° C. and +10° C.
 5. The methodaccording to claim 1 wherein said casing is produced from a plasticmaterial by a foam method.
 6. The method according to claim 5 whereinsaid plastic material is a foamable polyurethane.
 7. The methodaccording to claim 1 wherein said casing is produced from two differentplastic materials.
 8. The method according to claim 7 wherein said twoplastic materials are thermoplastic plastics.
 9. The method according toclaim 1 wherein in said central region for an airbag covering, saidcasing is formed with a reinforcement insert of one of a plastic fabric,a plastic net, a perforated metal foil, and a plastic film, saidreinforcement insert being interrupted along said tear-open lines.